In order to obtain a finished ophthalmic glass suitable for mounting on a spectacle frame, a rough lens presenting a generally cylindrical rim is radially ground following a predetermined contour corresponding to that of the ring of the frame in which the glass is intended to be mounted.
In order to grind the lens, the profile of at least one of its faces is plotted at right angles to the contour to be formed, then the lens, rotated, is brought into contact by its rim with a grinding tool.
Once the grinding operation has been carried out, the lens presents a sharp edge at the junction of its rim and each of its faces.
These are the edges that it is wished to bevel.
In fact, the lens has at its rim a certain thickness, such that, once mounted in the ring of its frame, it protrudes on either side of the latter. It is thus wished to visually soften the junction between the ring of the frame and each of the faces of the lens.
Moreover, taking account of the curvature of the faces of the lens, its edges can prove to be sharp, which is dangerous in itself.
Moreover, a lens that is not bevelled is very fragile at its edges, and it very often happens that small splinters become detached from it, which could injure the wearer and give the lens an unattractive appearance.
In practice, to bevel a lens, the lens is rotated, then a bevelling tool is brought into contact with each of the edges.
Generally, an ophthalmic lens does not display symmetry in revolution but has a certain curvature that is generally compensated for, during the bevelling, by displacing the tool parallel to the rotational axis of the lens during the rotation of the latter.
The quality of a bevel is assessed by measuring its value “across the flat”, i.e. the width of the oblique surface thus obtained.
A bevel is said to be of good quality if its value across the flat is constant or approximately constant (variations of the order of 10% which are undetectable to the naked eye can be tolerated) over the whole periphery of the lens.
A bevelling process is known in which a tool is used containing means of compensation permitting a certain elastic deformation of the tool in order to apply it in the most uniform manner possible to the worked glass.
This process, which was made the subject by the applicant of a French patent application published under the number FR-2 811 599, is entirely satisfactory, in particular as regards its economics and the relatively good quality of the bevel that it allows to be obtained at lower cost.
However, while exploring new routes, it is desired to further improve the quality of the bevelling using a more systematic approach.
It is known that, for a constant given angle of bevel, the value across the flat of this bevel depends, at each point, on the curvature of the lens at this point. This can be demonstrated by geometry.
Specifically, a bevelling process is known in which the local curvature of the face of the lens is plotted by means of a mechanical sensor, in the vicinity of the edge to be bevelled and in which, during the bevelling of the lens, the latter is rotated, and the bevelling tool is displaced parallel to the rotational axis of the lens according to the plots produced.
Although this process permits a good-quality bevel to be obtained, it nonetheless presents a certain number of drawbacks, listed below, which it is wished to remedy.
The mechanical force applied by the sensor to the lens engenders, on the sensor or on the lens, stresses and strains which can lead to measurement errors during the plots.
The friction engendered by the sliding of the sensor on the lens during its rotation can produce unattractive scratches on the relevant face of the latter.
This same friction causes chronic wear on the sensor, which, on the one hand, leads to measurement errors during plotting and, on the other hand, necessitates a frequent replacement of the sensor or, in the best of cases, a calibration of the sensor.
Taking account of the curvature of the relevant face of the lens, the reaction of the lens applied to the sensor contains a component perpendicular to the axis of the sensor, which can lead to a bending, or at least an eccentricity of the sensor and, thus, induce not only a measurement error, but also damage to the sensor.
When the sensor is situated, at least locally, next to the rim of the lens, it tends to slide towards the outside of the latter and thus runs the risk of being damaged.
The aggressive machining environment, where water and grinding splinters mix, causes accelerated wear on the sensor which, moreover, is a mechanical assembly which is tricky and costly to realize.
As the sensing is punctiform, it is necessary, in order to obtain the curvature of the lens in the vicinity of the edge to be bevelled, to produce two successive plots at different radii for each face, the curvature being defined, for each point of the edge, by the inclination, with respect to the rotational axis of the lens, of the straight line connecting the two corresponding plotted points.
Two complete turns of the lens are therefore necessary for each of its faces, which increases the cycle time.